If you're talking about "amp-in-a-box" pedals that can do mid to high gain, then the ones I've used that can replace an amp's gain are Bogner Red, Blackstar HT drive and Carl Martin Plexitone.
Of those, the Bogner is my favorite. Most versatile and just kills on a good clean channel. Very similar in tone and feel to a real Ecstasy (had both)
The Blackstar HT drive is a really fat and warm medium gain pedal that approaches high gain crunch. Sounds sort of Marshall inspired. Really a fun pedal and definitely feels like you're playing an extension of your amp. It basically gives your amp another channel.
Carl Martin plexitone - nice pedal. It's not very similar to a real plexi at all because of the CRAZY amount of distortion on tap but has certain shades of Marshall tone. With the gain dialed way back you can get a nice crunch tone. My only complaint is that it's too focused in the low and high-end and not enough in the mids. It's a little "cold" sounding and boomy but overall, the amp-like feel is there.
For natural overdrive, my favs are JRockett Archer and Xotic BB. I've heard the Kingsley pedals and some of the JHS pedals are great such as the Angry Charlie, Supro and Silvertone amp pedals.